1. Field of the Invention
This invention relates to methods of manufacturing semiconductor devices, and more particularly to forming ultra shallow junctions in such devices.
2. Related Art
Charge Coupled Device (CCD) image sensors have been utilized in a variety of vision systems for a wide range of applications including highly demanding ones such as high sensitivity spectroscopy, high sensitivity chemical analysis and proteomics, high throughput drug screening, high throughput industrial inspection, high speed x-ray crystallography and any other high frame rate low light level applications. The most challenging applications are those that require high speed (also referred to as high frame-rate) in extreme low illumination levels. This type of imaging presents a set of difficulties not encountered in conventional applications such as digital still cameras or digital cinema. The photon-starved environment stresses all performance characteristics of the sensor at high frame rates. In order to accommodate the high frame rate, the pixel rate must increase, which consequently increases the noise bandwidth of the CCD output amplifier and thus leads to an increase in the readout noise of the sensor. This will result in reduction in the Signal-to-Noise-Ratio (SNR) of the device. Furthermore, as the frame rate increases, the integration time per frame decreases, causing additional degradation of the device SNR.
There are currently two technologies that address imaging applications of high speed in low light level, namely Image Intensifier (II) technology and Charge Multiplication CCD technology. Both technologies suffer from drawbacks that limit their SNR.
The Image Intensifier was originally developed for military use and dominated low light level imaging for decades. It has an input photocathode followed by a micro-channel plate electron multiplier and a phosphorescent output screen. The gain of the micro-channel plate is adjustable over a wide range, with a typical maximum of about 80,000 photons pulse from the phosphor screen per one photon input. The multiplied photons are then sensed by the CCD or CMOS imaging sensor. By amplifying each photoelectron by a gain as high as 100,000, the device essentially eliminates the readout noise of the CCD or CMOS imaging sensor.
However, the technique suffers from several drawbacks. For example, Image Intensifier devices suffer from increase in Fixed Pattern Noise (FPN) due to the non-uniformity of the photoelectron gain of the device across the entire imaging area. That causes reduction in SNR and increases device complexity for FPN correction functionality.
Further signal degradation is caused by the gain uncertainty for each interaction (referred to as electron multiplication noise) that manifests similarly to Shot Noise. This effect is characterized by the “noise factor” parameter (NF). A typical best case NF value is ˜1.7. The NF has the equivalent effect of lowering the Quantum Efficiency (QE) of the device by the square of NF. Thus, an Image Intensifier device with a native QE of 45% and best NF of 1.7 will be reduced down to 15.571%.
An additional problem is the limited bandwidth of the spectral response of the Image Intensifier device, which limits sensitivity to the longer red wavelengths, V, and deep blue, a characteristic that is often not ideal for a CCD and thus not desired.
Furthermore, an Image Intensifier device suffers from relatively low intra-frame dynamic range unless it encumbers extra device complexity. It is difficult to obtain more than a 256-fold intensity range from the Image Intensifier device. Dynamic range expansion can be achieved via a gated variable gain intensified CCD that results in a more complicated device.
A Charge Multiplication CCD device is a conventional CCD structure extended with an additional charge transfer control section that provides voltage level (e.g., 40 Volts) that is significantly higher than conventional levels (e.g., 10 Volts). Thus, electric fields in the semiconductor material are created that accelerate the charge carriers to sufficiently high velocities so that additional carriers are generated by impact ionization (also referred to as avalanche gain). The probability of charge multiplication per transfer is quite small (e.g., 1%) but with a large number of transfers, substantial electronic gains may be achieved. For mean gain per stage R and n number of transfers, the total gain G=(1+R)n. The maximum gain per stage R is typically 0.015 as set by the onset of excess noise. If n is high enough, the effective output read noise is reduced to very low levels (e.g., <1 e− rms) since the output amplifier electronic noise (e.g., 100 e− at 1 MHz pixel rate) is divided by the gain factor of the multiplication register.
However, though the charge multiplication CCD device offers much higher quantum efficiencies as compared to the Image Intensifier device, it suffers from several serious drawbacks.
One problem is the noise which is caused by the uncertainty in the actual gain and is the same as for the Image Intensifier device (referred to as noise factor NF=1.414 to 1.6). This noise appears similar to Shot Noise and degrades the SNR of the device.
Furthermore, the technique requires extra circuit complexity for a very fine control of the high amplitude clock pulse. This fine control is required since the multiplication gain is a very strong function of gate clock voltages such that any variation in the clock rails will have a serious effect on the Noise Factor of the device. For example, a typical gain needed for effective noise reduction is G≈100, and a 1V error in the clock voltage will produce a 500% error in the gain.
In addition further complexity is required for overall system control due to the fact that the avalanche gain (i.e. impact ionization) is an exponential function of temperature, and thus has very strong temperature dependence. Hence, a small temperature variation can produce a large change in the register gain (e.g., a variation of ˜1° C. produces a ˜5% change), stressing the temperature control of the system.
Accordingly, it is desirable to have a CCD that can be used in high frame rate, low light level conditions without the disadvantages discussed above associated with CCDs or imaging sensors.